For an analogy, look up brownian motion. http://en.wikipedia.org/wiki/Brownian_motion http://en.wikipedia.org/wiki/Brownian_motion All those molecules bumping

For an analogy, look up brownian motion. http://en.wikipedia.org/wiki/Brownian_motion http://en.wikipedia.org/wiki/Brownian_motion All those molecules bumping into a particle do cancel each other out over a long time. However, particles do drift noticeably due to this phenomenon.